(1) Field of the Invention
This invention relates generally to current sensing and relates more specifically to fully integrated bidirectional current sense as e.g. currents into and out of a battery.
(2) Description of the Prior Art
Sensing bidirectionally a battery current into and out of a battery by an integrated circuit usually requires an external resistor, which transforms the current into a voltage that then can be measured with a precision analog-to-digital converter (ADC). Since this resistor is placed in the high current path it has to be of low value because of the associated voltage drop. The value of this resistor is usually in the order of 100 m Ohms or less.
This leads to very small voltages, which have to be sensed. These small voltages put tough requirements on the specifications for the ADC and its input amplifiers in terms of noise, offset and dynamic range. These and the additional voltage drop (especially at high currents) are the main disadvantages of state-of-art implementations.
Therefore it is a challenge to designers of circuits to sense currents without using a sense resistor.
Solutions dealing with sensing of currents are described in the following patents:
U.S. patent (U.S. Pat. No. 7,327,149 to Chapuis) discloses a current sensing circuit comprising a power device adapted to conduct a bidirectional current between first and second terminals thereof, first and second sensing devices operatively coupled to the power device, a sense amplifier providing first and second voltages to the first and second sensing devices, and a gate drive device providing activating signals to the power switching device and the first and second sensing devices. The first and second sensing devices each has an active area that is substantially identical and significantly smaller than a corresponding active area of the power-switching device. The sense amplifier measures the voltage of the first sensing device and maintains the voltage on the second sensing device at the same level as the first sensing device by injecting an additional current into the second sensing device. The sense amplifier further provides an output signal proportional to the bidirectional current. The first and second sensing devices have k times higher resistance than a corresponding resistance of the power device when in an active state.
U.S. patent (U.S. Pat. No. 5,495,155 to Juzswik et al.) proposes an improved current sensing circuit provided in, or for use with, a power delivery circuit having a current controlling device of the type, which includes auxiliary current sensing terminals. The current sensing circuit is particularly useful in conjunction with a respective current sensing power MOSFET located in the “high side” of a switching circuit, such as an H-bridge switch for a bidirectional load. The current sensing circuit has two branches connected to the current sensing MOSFET and to a reference potential and constructed to provide an output voltage signal representative of the main current through the current sensing MOSFET. The first branch includes at least two transistors and a resistor serially connected between a current sense terminal of the MOSFET and a reference potential. The second branch also includes at least two serially connected transistors connected between a terminal of the MOSFET and the reference potential. The transistors are cross connected between branches and provide an inverted current mirror and a current mirror. Fifth and sixth transistors may also be included as part of the current sensing circuit.
U.S. Patent Application Publication (US 2008/0197801 to Manor et al.) proposes a bidirectional battery charge control system for a portable electronic device, which uses a rechargeable main battery. The system enables the connection of an auxiliary battery to the device for inputting additional current to the device. Control of the current flow into and out of the auxiliary battery is performed by a bidirectional charger. The auxiliary battery can contain one or more readily available primary or secondary cells, and the bidirectional charger is such that an external charger connected to the device, generally used to charge the main rechargeable battery of the device, can also recharge a secondary cell or cells in the auxiliary battery, if such are installed. The use of such an auxiliary battery enables the main battery to be hard-wired into the device.